Spark plug



June 15, 1937. J J MASCUCH AL 2,083,916

SPARK PLUG Filed June 12, 1935 2 Sheets-Sheet 1 9 70 dost-55 filg llC/f [DU/J W- 6. FLY/V7 I GARDNER June 15, 1937 J. J. MASCUCH El AL SPARK PLUG Filed June 12, 1935 2 Sheets-Sheet 2 flrlialvii 7 Patented June 15, 1937 UNITED STATES PATENT OFFICE SPARK PLUG tion of New Jersey Application June 12, 1935, Serial No. 26,096

8 Claims.

This invention relates to spark plugs.

In the past considerable difficulty has been experienced with spark plugs in service by reason of the fact that under high temperatures the plugs are not gas tight. Where leaks occur the efiiciency of the spark plug is materially reduced. This is particularly noticeable where the spark plugs are used on aviation engines. The broad object of this invention is to provide a spark 10 plug which will be gas tight and maintain its efficiency for long use under heavy stresses such as are found in aircraft practice.

One cause of the failure of spark plugs is the' releasing of pressure between the sealing surfaces of the laminated mica insulation and the metal components that support them, due to relatively large differences in the thermal expansion characteristics of the two materials, which permits the invasion of gases at high temperature and pressure between the laminae and other contacting surfaces, the-said invasion carrying with it uncombined carbon in finely divided solid form which by its electrical conductivity reduces the ohmic resistance of the insulation to a point below that established at the sparking gap provided. Therefore, a further object of this invention is to provide a spark plug the essential metallic parts of which will have thermal expansion characteristics comparable to that of the insula- 'tion material employed and thus maintain the pressure required for sealing over the entire operating temperature range.

In order to have a gas tight spark plug over the entire temperature range encountered during service, it is necessary to provide a plug which will be gas tight at normal temperatures and will remain so at high temperatures. Therefore, a further object of this invention is to provide a method of assembling a spark plug in order to achieve an efficient result. A further, specific object of the invention is to provide means to apply both an axial and a radial load to the parts in the assembly of the spark plug to offectively seal the parts together. By providing effective sealing between the parts the service life of the spark plug is greatly lengthened.

Overheating of the spark plug causes rapid deterioration and consequent failure. It is essential that the heat which accumulates in the spark plug be readily dissipated to prevent overheating. Therefore, a further object of the invention is to provide a spark plug so constructed as to permit the rapid dissipation of accumulated heat. A specific object of the invention is to pro- 65 vide a construction wherein a single metallic member is in direct contact with the outer portion of the spark plug to permit direct conduction of heat from the center electrode of the spark plug to the outer casing. A further specific ob- Ject of the invention is to provide a spark plug in which the cross-sectional area of the center electrode is reduced in the upper portion so as to retard the transmission of heat to the upper part of the spark plug, thereby minimizing the temperature at the point where the contact between the conductor cable and the spark plug terminal is made in order that temperatures damaging to the cable insulation may not be transmitted to that point.

Where radio equipment is used in proximity to an internal combustion engine it is necessary to shield the ignition system by surrounding the system with grounded conductive casings to prevent interference with the radio equipment. This shielding must extend to and include the spark plugs. Therefore, a further object of the inven-'- tion is to provide a spark plug adapted to receive radio shielding elements.

A further object of the invention is to provide a radio shielding means for a spark plug of the type herein described.

Where radio shielding has been used on spark plugs considerable difficulty has been experienced by reason of the overheating and charting of the insulation surrounding the spark plug lead with resultant deterioration and eventual breakdown. Where metallic covered dielectric caps have been used over the upper part of a spark plug,- experience has shown that frequently the insulation of the conductor will become in effect vulcanized to the dielectric of the cap, thereby preventing ready separation of the parts for the purpose ofservicing. Therefore, a further speciflc object of the invention is to provide a radio shielding cap for a spark plug having means to space the spark plug conductor from the dielectric of the cap to prevent the insulation of the conductor adhering to the cap, and also to prevent overheating of the cap and conductor.

In the past considerable difficulty has been experienced by reason of overheating and charring of the insulation of the conductor at the point where the contact means from the spark plug makes contact with the conductor. Therefore a further specific object of the invention is to provide a contact means at an end of the conductor which will insure a good contact and at the same time preclude the possibility of burning the insulation of the conductor.

The invention consists oi the construction,

scribed and claimed.

In the accompanying drawings, formingpart hereof, is illustrated one form of embodiment of the invention, in which drawings similar reference characters designate corresponding parts, and in which:

Figure 1 is an enlarged vertical section the spark plug;

Figure 215 an enlarged vertical section through the sealing bushing and compression hoop before the application thereof to the spark plug;

. Figure 3 is a vertical section, partly in elevation, showing the spark plug with the radio shielding cap applied thereto;

Figure 4 is a vertical section. tion, of the shielding cap; and,

Figure 5 is a bottom plan view, on a smaller scale, of the plug shown in Figure l.

Referring to the drawings, the outer part of the through partly in elevaspark plug comprises a shell 6 provided with extemal screwthreads l for engagement in an appropriate opening in a cylinder head. The lower part of theshell 8 is formed with an inwardly curved base I having formed therein a plurality of slots 0 to provide a plurality of sparking points or areas It. The shell 6 is provided with an internal shelf or shoulder ii and the upper part of the shell 2 is provided with internal threads i 2 to receive a retaining means as hereinafter described. A washer it may be disposed adjacent the threads I to insure a gas tight seal between the shell 6 and the cylinder head of the engine which receives the spark plug.

Disposed in the shell 8 is a center electrode ll provided with a tip IS. The electrode I4 is formed of a metal having an exceedingly low or practically negligible co-efllcient of expansion and the tip Ii may be formed of metal adapted to withstand corrosion and erosion in service. The electrode l4 and the tip l5 may be made of a single piece of metal having a negligible coefllcient'of expansion rather than of two pieces. The electrode it may be coated with a material of greater heat emissivity than the metal of the electrode It, to more readily permit conduction of heat from the electrode H to the outer part of the spark plug. The tip I! is formed with a shoulder II to receive the sealing means hereinafter described.

Disposed around the electrode l4 and the upper a stack of mica washers I! which rest on the shoulder I of the tip ll. Centrally of the plug and above the washers i8 is disposed a sealing bushing II formed with an integral flange having a tapered face 2i adapted to bear against the internal shoulder ll of the shell I. The bushing ll may be made of copper or other metal suitable for rapid conduction of heat.

Surrounding the bushing is is a compression hoop 22 formedof a metal having a negligible co-efiicient of expansion similar tothe electrode ll. The lower portion of the hoop 22 is formed with a flange 22 overlying the flange 2| of the bushing ll.

Above the bushing i9 and hoop 22 and surrounding the tube I1 is a second stack of mica washers 24.

Above the washers 2| and surrounding the upper end of the electrode I 4 is a terminal 2| provided with an annular groove 22 to receive a contact means to supply current to the electrode ll. The upper portion of the electrode II is formed with a bore 21 which extends to a point adjacent the top of the bushing it. The purpose of the bore 21 is to reduce the crosssectional area of the upper portion of the electrode ll to retard the passage of heat to the upper part of the spark plug. When the spark plug is assembled the top of the electrode I4 is turned over at 28 against the terminal to hold the parts together as hereinafter described. A compressible washer 28 is disposed on the flange 22 of the hoop 22 and the electrode it with its associated parts is held in the shell 6 by a retaining cap 20 having a lower threaded portion 3i adapted to engage with the internal threads i2 of the shell 6. The lower part of the retaining cap it bears against the washer 29 which serves the purpose of equalizing the pressure at all points on the flange 23. The upper portion of the retaining cap 20 is formed with threads 32 to provide for the optional application to the spark plug of a radio shielding cap ashereinafter described.

In the manufacture of the spark plug the electrode i4 and the tip ii are first formed. If a two piece construction is used, the tip I! is welded to the electrode l4 and the parts are machined to exact size to form the shoulder It. If a one piece construction is desired, the electrode l4 and the tip are machined directly from a single blank. The bore 21 is then formed in the electrode H which is now ready to receive its associated elements.

A sheet of mica is then wrapped around the electrode It to form the tube l1 and the lower stack of mica washers i8 is applied thereover. The sealing bushing l9 andthe compression hoop 22 are next applied and when first applied to the electrode It the bushing l9 and the hoop 22 are in the shape shown in Figure 2. The two members may be applied as a sub-assembly or may be applied separately. In either case, before application, the bushing l9 and the hoop 22 are stressed outwardly by means of an expanding drift so that when these members are reduced by swaging as hereinafter described, there will be no tendency on the part of the metal to again expand. The upper stack of mica washers 24 is then applied, followed by the terminal 25. when these parts have been assembled they are placed in an appropriate fixture and an axial load is imposed on the terminal 25 to eifectively seal all of the elements surrounding the electrode II together. While the parts are maintainedunder the axial load the upper end of the electrode I4 is turned over at 28, against the terminal 25 to permanently retain the parts in sealed relation. In the assembly thus far the tube II has not been distorted.

The bushing l9 and the hoop 22 are then reduced by passing the entire assembly through a swaging die. As shown in Figure 2, the flange 20 of the bushing I9 is provided with a slightly tapered face 2i. The face 2| is so tapered to avoid drag on the swaging die which would result if the face 2i were square. The swaging of the members I! and 22 compresses the tube H, as shown in Figure 1, and forces the central portion of the bushing I 8 closer to the electrode I 4. The radial load .thus imposed causes an effective sealing of the several parts and causes a slight elongation of the members I! and 22, which results in an increase in the axial load already imposed on the parts, and incldentallyestablishes an initial axial stress approximating the elastic limit of the central electrode material thus maintaining the necessary pressure between the mica laminae and adjacent metallic parts toinsure the maintenance of a permanent gas tight seal under all normal operating conditions. This forms a permanent gas tight seal between the several parts, and since the electrode 14 and the hoop 22 are formedof metals having a negligible co-eflicient of expansion, there is no possibility of leakage under the high temperatures encountered in service. At the same time, since the bushing 13 and the hoop 22 were initially expanded' before being swaged, they are in practically a normal rather than stressed condition after being swaged, so that there is no tendency to expand. a

Where a single sheet of mica is used to form the tube l'l there may be very minute interstices left at the ends of the sheet. In a horizontalsection these interstices would appear substantially as triangular spaces, the base of the triangle being the thickness of the mica sheet. As

order to prevent the possibility of leakage through these spaces longitudinal of the tube l I, it is advisable to effectively seal the junctions between the several sheets of mica used. or where a single sheet of mica is used, to seal the junctions between the ends of the sheet and the center electrode l4, and the sealing bushing l3, respectively. The sealing may be accomplished by the use of a suitable filler or sealing agent which will fill in minute interstices formed as above described.

The sealing may be accomplished in the following manner: After the tube I1, the bottom washer assembly i3, and the sealing bushing l9 have been applied to the electrode l4, and

, before the application of pressure as above described, the sealing agent may be incorporated. The sealing agent may be rendered liquid through heating and incorporating by saturation through inunefsi'on. The sealing agent may be forced into the interstices under pressure in order to assure complete filling. If an effective seal is achieved over the area adjacent the sealing bushing is it is not necessary to apply the filler above-the bushing it. However, if desired. the filler or sealing agent may be applied after .the washer assembly 24 and the cap 23 are applied, but before the application of pressure.

When the sealing agent has been incorporated the parts. are subjected to the axial load hereinbefore described and also subjected to the radial load. The pressure thus applied will extrude any surplus volume of the sealing agent and effectively eliminate the possibility of leakage through any interstices which may have existed adjacent the ends of the sheets of mica used to form the tube i1.

The electrode i4 with its assembled parts is then machined to form the mica washer stacks i3 and 24 to the contour shown and to present a smooth finish. At the same time the compression hoop 22 is machined from the shape shown in Figure 2 to the shape shown in Figure 1 to provide the flange 23. The tapered face 2! of the flange 20 on the bushing I3 is also machined.

to form the face 2| at an angle complementary to the angle of the shoulder H, so that the two parts will lie flat together. The parts are now ready to be assembled with the shell 3-, and when the washer 23 has been applied the electrode l4 and its associated parts are inserted in the shell 3 and held in place in the obvious manner by the retaining cap 30.

When it is desired to apply radio shielding to the spark plug, such shielding may be applied as shown in Figures Band 4. A metallic cap 33 is provided and adapted to be placed over the upper end of the spark plug. The cap 33 may be in the form of an elbow as shown, or may have a different shape. The lower edge of the cap 33 is formedwith a bead 34 which is adapted to be engaged by acoupling nut 33 formed with a flange 33 which is turned over the bead 34. The nut 33 is free to rotate on the cap 33 and is adapted to engage the threads 32 on the upper part of the retaining member 30 while the lower skirt of the cap 33 contacts themember 33. In this way a good electric contact is made and the cap 33 may be readily removed for the purpose of inspecting or servicing the spark plug.

- The cap 33 is provided with a dielectric lining 31 having a thickness sufllcient to effectively insulate the terminal 23 and the contact means hereinafter described from the cap 33. The upper portion of the lining 31 is formed with a bore 38 to receive a conductor with its insulation. In order to prevent the insulation of the conductor from being vulcanized to the walls of the bore 33 by reason of the high temperatures encountered, the walls of the bore 33 are formed with ribs 33 which space the insulation of the conductor from the walls and provide a passage for the circulation of air.

Carried by the lining 31 is a threaded bushing 40 in which is threaded a contact screw 4| to the lower end of which is connected 9. spring 42 adapted to make contact with the terminal 23 of the spark plug. The upper end of the screw 4| is provided with a point 43 adapted to-make contact with the conductor.

A conductor 44 surrounded by insulation 43 is provided to supply current to the spark plug. In the past, it has been the practice to extend the insulation 43 to the left hand end of the bore 33 andthe point 43 of the screw 4| was used to pierce the insulation 43 to make contact with the conductor 44. This practice causes burning of the insulation 43, which is undesirable. In order to avoid this condition the conductor 44 is extended to the end of the bore 33 but the insulation thereof 43 is removed for an appreciable distance, as shown in Figure 3. A coil spring 43 is connected to the conductor 44 and surrounds the conductor. The contact screw 4| extends partially through the coil spring 43 to make an effective electrical contact by reason of the resilience of the spring 43, and at the same time the danger of burning the insulation 43 is eliminated.

At the point where the conductor with its insulation 43 leaves the cap 33, a conical packing 41 provided with a copper or other metallic re-inforcing means 43 is disposed. The packing 41 may be of the type shown in co-pending application, Serial No. 731,833, flied June 22, 1934,

. and not only provides a waterproof joint, but the metallic portion 43 serves to improve the electrical contact between the cap 33 and a radio shielding conduit 49 which surrounds the conductor N with its insulation 45. The conduit 49 is provided on its end with a ferrule 59 formed with a flange SI, and a coupling nut 52 serves to hold the conduit 49 against the cap 33 by reason of engagement with threads 53 formed on the cap 33. The pressure of the ferrule 59 against the packing 41 has a tendency to force the packing ll inwardly against the insulation 45, thereby holding the conductor 44 firmly in place and removing any strain from the end oi the conductor, which eliminates the possibility of breakage of the conductor 44 at the point of.

contact with the screw ll.

The general structure of the shielding cap and contact means may be the same as shown in co-pending applications Serial No. 481,766, filed Sept. 13, 1930, and Serial No. 657,166, filed Feb. 17, 1933. The structure of the conduit may be the same as shown in co-pending application Serial No. 657,167, filed Feb. 17, 1933.

Having thus fully described the invention. what is claimed as new and desired to be secured by Letters Patent is:

1. A spark plug comprising a shell, an electrode supported in the shell, a tube of insulation disposed around the electrode, a sealing bushing disposed centrally oi the electrode and in intimate thermal contact with the insulation, said bushing having formed thereon a flange in contact with the shell, additional insulating material around the electrode above and below the bushing, a compression hoop surrounding the.

bushing and a retaining means to hold the parts in the shell.

2. A spark plug comprising a shell, an electrode supported in the shell, a tube of insulation disposed around the electrode, a sealing bushing disposed centrally of the electrode and in intimate thermal contact with theinsulation, said bushing having formed thereon a flange in contact with the shell, additional insulating material around the electrode above and below the bushing, a compression hoop surrounding the bushing and a retaining means to hold the parts in the shell, said retaining means being formed to receive aradio shielding means.

3. A spark plug comprising a shell, an electrode supported in the shell, a tube of insulation disposed around the electrode, a sealing bushing disposed centrally of the electrode and in intimate thermal contact with the insulation, said bushing having formed thereon a flange in contact with the shell, additional insulating material around the electrode above and below the bushing, a compression hoop surrounding the bushing and a retaining means to hold the parts in the shell, said electrode being formed with a bore to reduce the cross-sectional area of the electrode above the bushing.

4. A spark plug comprising an electrode of metal having a negligible co-efl'lcient of expansion, a body of insulation around the electrode, a sealing bushing around a portion of the insulation, said bushing being formed of metal adapted to readily transmit heat and formed with a flange to dissipate heat directly to the outside of the plug, a compression hoop formed 01' metal having a negligible co-efllcient of expansion around the bushing, a shell in which the foregoing elements are supported and a retaining means co-operating with the shell to hold the elements in the shell.

5. A spark plug comprising an electrode of metal having a negligible co-eflleient otexpansion, a body of insulation around the electrode, a sealing bushing around a portion of the insulation, said bushing being formed of metal adapted to readily transmit heat and formed with a flange to dissipate heat directly to the outside of the plug, a compression hoop formed of metal having a negligible co-efllcient of expansion around the bushing, a shell in which the foregoing elements are supported and a retaining means co-operating with the shell to hold the elements in the shell, said electrode being formed with a bore to restrain the transmission 01 heat above the sealing bushing.

6. A spark plug comprising an electrode of metal having a negligible co-efllcient of expansion, a body of insulation around the electrode, a sealing bushing around a portion of the insulation, said bushing being formed of metal adapted to readily transmit heat and formed with a flange to dissipate heat directly to the outside or the plug, a compression hoop formed of metal having a negligible co-efllcient of expansion around the bushing, said compression hoop being in close thermal contact with both said bushing and the outside member of the plug to permit ready dissipation of heat, a shell in which the foregoing elements are supported and a retaining means cooperating with in the shell.

7. A spark plug comprising an electrode of metal having a negligible co-efllcient of expansion, insulation in the form 01' a tube made from a sheet or sheets of mica wrapped around the electrode, the interstices adjacent the ends of the sheet or sheets being filled with a sealing agent to prevent leakage, a sealing bushing around a portion of the insulation, said bushing being formed of metal adapted to readily transmit heat and formed with a flange to dissipate heat! directly to the outside of the plug, hoop formed of metal having a negligible co-efllcient of expansion around the bushing, a shell in which the foregoing elements are supported and a retaining means co-operating with the shell to hold the elements in the shell.

8. A spark plug comprising an electrode of metal having a negligible co-eillcient of expansion, a coating on the electrode to increase the thermal emissivity of the electrode, a body of insulation around the electrode, a sealing-bushing around a portion of the insulation, said bushing being formed of metal adapted to readily transmit heat and formed with a flange to dissipate heat directly to the outside of the plug, a compression hoop formed of metal having a negligible eo-eiflcient of expansion around the bushing, a shell in which the foregoing elements are supported and a retaining means co-operating with the shell to hold the elements in the shell.

the shell to hold the elements. 

